This invention relates to a novel polymer binder and more particularly to a propellant and a plastic bonded explosive binder comprising a polymer blend made of polyethylene glycol and poly(2-methyl-5-vinyl tetrazole).
Efforts are proceeding to improve propellents and explosives for fulfilling current ordnance needs, e.g., to produce an explosive having high energy with reduced sensitivity. By reduced sensitivity is meant thermal stability, increased resistance to cook-off and shock initiation. As an explosive is a composite of organic binders and crystalline nitramines, the binder component should be both tough and soft, especially in the time frame of an impact. The binder has to be soft in order to avoid the comminution of energetic materials which would cause a drastic increase of surface area resulting in greatly increased reaction rates and contributing to enhanced sensitivity. If the binder is hard, a fracture would propagate through the crystalline nitramines which would increase sensitivity. By making the binder soft, the crack propagates through the soft segments and the crack tip would not initiate the nitramines. If the binder is not tough, it will fail early in the impact event thereby exposing the nitramines to the threat.
An explosive binder material must remain in the plastic or non-glassy state throughout the military temperature specification range. Otherwise, it will be brittle rather than tough and soft, and will not protect the nitramines. This is only possible if the glass transition temperature (Tg) is very low.
We have found that the sensitivity may be improved if the percent of crystalline nitramine is reduced. We have also found that by using a moderately energetic material such as PMVT as one of the energetic binder components in the blend, it is possible to impart such insensitive characteristics to the explosives thus formed without reducing their performance.
Polyethylene glycol (PEG) and poly(2-methyl-5-vinyl tetrazole) (PMVT) are used separately in solid propellants and explosives as binders. A well recognized inherent property of PEG is that it is non-energetic. While PMVT, a stereoirregular syndiotactic polymer, has some energy, it is a glassy and somewhat brittle material. Both of these materials have deficiencies when used independently. PEG has low density, i.e., 1.1 g/cc, but has both hard and soft segments. PMVT is a little more dense, i.e., 1.28 g/cc. An ideal binder should have the following characteristics:
(a) energetic, tough and soft
(b) capable of wetting crystalline nitramine and other explosives, and
(c) capable of dissolving large proportions of plasticizers without exudation.
Attempts have been made to solve the deficiencies with a designer binder by co-polymerizing two or more component polymers, thereby, incorporating the characteristics of the constituent polymers. An example is the Kraton binder having widespread use in explosives. Kraton is a block co-polymer of styrene and butadiene. Butadiene has a low Tg, and provides the "soft" segment. Styrene has more crystalline brittle segments and has a high Tg. When the two polymers are converted to the block co-polymer Kraton, there is evidence of the characteristics of both polymers in the binder. While Kraton absorbs more than three times its volume of plasticizer without exudation, it possesses low density and does not wet explosives satisfactorily.